1. Field of the Invention
The present invention relates in general to a method and a device for molding cassettes, such as tape cassettes, disc cassettes or the like, which contain therein a recording tape or recording disc. More specifically, the present invention is concerned with a method and a device for molding cassettes of a type which comprise injection-molded plastic lower and upper cassette halves.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional method for molding a cassette of the above-mentioned type will be described with reference to FIGS. 8 and 9 of the accompanying drawings.
The cassette which will be described in the following is a tape cassette which comprises generally lower and upper cassette halves which are united to constitute a rectangular case for containing therein a recording tape. Each cassette half has a double-layered structure which includes a lower resiliency plastic layer (or inner layer) and a higher resiliency plastic layer (or outer layer) which are laid on each other. Due to the provision of the lower resiliency plastic layer, undesired vibration applied to the cassette is absorbed.
Referring to FIG. 8, there is shown a rotary type injection molding device 101 for molding halves of the tape cassette. The molding device 101 generally comprises a first fixed die 103, a second fixed die 106 and a movable die 104. First and second resin feeding cylinders 102 and 108 are respectively connected to the first and second fixed dies 103 and 106 for feeding them with respective materials for the lower and higher resiliency plastic layers. Each die shown in the drawing is shaped to have two rectangular cavities.
For producing the cassette half, the following steps are taken.
First, the movable die 104 is coupled with the first fixed die 103, and then, the first resin feeding cylinder 102 injects the material for the lower resiliency plastic into a shaped cavity defined by the coupled dies 103 and 104. With this, the lower resiliency plastic layer 105 is molded in the coupled dies 103 and 104. Then, the movable die 104 is disengaged from the first fixed die 103 and coupled with the second fixed die 106 and then the second resin feeding cylinder 108 injects the material for the higher resiliency plastic into the newly coupled dies 106 and 104. With this, the higher resiliency plastic layer 107 is molded or laid on the lower resiliency plastic layer 105.
According to the above, upon completion of the injection molding by the coupled dies 103 and 104, the movable die 104 is shifted from the first fixed die 103 to the second fixed die 106 having the injected lower resiliency plastic layer 105 kept in the movable die 104. As seen in FIG. 9, in order to keep the plastic layer 105 in the movable die 104 during such shifting, the movable die 104 has from a bottom wall a rectangularly extending retaining ridge 109 having a dove-tail cross section. Therefore, upon injection molding of the material for the lower resiliency plastic layer 105, the retaining ridge 109 is immersed in the injected material 105 in a manner to retain the same (viz., hardened plastic layer 105). To assure the retaining function of the retaining ridge 109, it is preferable to increase the inclination ".theta." of each side wall 109a of the retaining ridge 109. Thus, the retaining ridge 109 has a so-called "dove-tail" shaped cross section.
However, the retaining ridge 109 having the above-mentioned construction brings about the following drawbacks.
That is, after molding the higher resiliency plastic layer 107 on the lower resiliency plastic layer 105, it becomes necessary to release a molded article, that is, a double-layered cassette half, from the movable die 104. Thus, due to the removal, the lower resiliency plastic layer 105 is forced to have an elongate groove (no numeral) therein as is shown in FIG. 9, which is caused by the retaining ridge 109. However, as is seen from the same drawing, because of the dove-tail cross section possessed by the retaining ridge 109, the lower resiliency plastic layer 105 is forced to have burrs 111 (viz., protruded lips) around the elongate groove, which lower the smoothness of the inner surface 105a of the cassette half. As is known, when the inner surface 105a of the tape cassette is not smoothed, smoothed movement of a tape in the cassette is not expected. Of course, when the inclination angle ".theta." of the side wall 109a of the projection 109 is reduced, the undesirable burrs 111 can be reduced in size. However, in this case, the retaining function of the retaining ridge 109 to the lower resiliency plastic layer 105 is sacrificed.